Surface treatment/coating technologies have been used to provide characteristics that iron materials does not have. Wet surface treatment technologies represented by electroplating and hot plating have been widely used in the field of surface treatment of steel, but there have been attempts to apply and incorporate dry surface treatment technologies such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) in the early 1980s. It has been known that it is difficult for conventional PVD and CVD processes to apply to the field of steel productions in consideration of workability and productivity associated with a coating rate and a thin film forming rate. However, developments for high-speed plating technology have been recently promoted by a group of advanced steel makers to secure productivity and cost leadership.
Steel sheets that have been used as the steel sheets for automobiles may be mainly divided into three groups: an electro-galvanized steel sheet, a hot-dip galvanized steel sheet and a galvannealed steel sheet. The electro-galvanized steel sheet has been used for an outer body plate for automobiles since it has excellent surface appearance. However, the electro-galvanized steel sheet has been decreasingly used because it is not favorable in terms of the workability in post-plating process, the manufacturing cost and the surrounding conditions.
The hot-dip galvanized steel sheet is cheaper than the electro-galvanized steel sheet in terms of the manufacturing cost, but has inferior properties, such as mechanical properties, coating adhesion in a molding process, spot weldability of electrode in a continuous impact test, to the electro-galvanized steel sheet due to the post-plating process. Also, the galvannealed steel sheet has superior properties, such as coating adhesion in a painting process and spot weldability of electrode, to the electro-galvanized steel sheet and the hot-dip galvanized steel sheet since a Fe—Zn-based inter-metallic compound is formed by an alloying reaction of zinc with base iron. However, a powdering problem associated with peeling-off of a coating layer from a steel sheet in a processing process may arise due to the presence of Fe—Zn-based alloy phase (gamma phase) formed during the alloying reaction.
Also, when a sealer used for waterproof purpose, corrosion prevention, vibration absorption and welding is pasted onto a steel sheet, the cohesive failure of an adhesive where the sealer is peeled off in an impact peel test and a shear strength test does not occur after the adhesion of the steel sheet by the sealer, but the cohesive failure of a coated steel sheet in which a Zn—Fe coating layer is peeled off may occur due to the alloy phase formed between Fe and Zn. Therefore, there is an urgent demand for development of a coated steel sheet satisfying all the properties such as moldability, spot weldability, paintability and corrosion resistance.
However, the above-mentioned coated steel sheet has difficulties in satisfying all the requirements that the steel sheets for automobiles should have. Therefore, vacuum deposition processes (i.e. resistance heating, magnetron sputtering, electron gun, ion plating processes) are used in addition to the conventional coating processes (i.e. electro-galvanizing, hot-dipping processes) to coat and alloy a thin coating material. However, the deposition processes have problems associated with slow deposition rate, low deposition yield, and low energy efficiency.
As the known technologies to obtain the above-mentioned coated steel sheet, there are Japanese Patent Laid-open Publication Nos. 1996060342, 1996134632, 1997-078229 and 1998-317125. These prior-art patent literatures disclose technologies of preparing a Zn—Mg alloy coated steel sheet having excellent adhesion to a coating layer and high corrosion resistance by depositing Zn onto a steel sheet using resistance heating deposition, depositing Mg onto the Zn-deposited steel sheet and heat-treating the Zn/Mg-deposited steel sheet. In this case, the resultant Zn—Mg alloy coated steel sheet has improved adhesion to a Zn—Mg alloy coating layer due to the presence of a Zn—Fe alloy layer or Zn—Fe—Mg alloy, and also shows its processability and corrosion resistance. However, these technologies disclosed in the prior-art patent literatures are not desirable in terms of economical efficiency due to the demand for the various deposition and alloying processes, and the cohesive failure of the coating layer may be caused in the sealer adhesion test due to the presence of the alloy phase formed between Fe and Zn during the alloying process. As another known technology, European Publication No. 0 756 022 discloses a method for preparing a zinc-based alloy coated steel sheet by depositing a metal element such as Fe, Mn, Cu, Mg and Ni onto a hot-dip galvanized or electro-galvanized steel sheet with a thickness of 0.1 to 3 micrometers (μm) by using electron-beam evaporation or magnetron evaporation, and alloying the metal element and the hot-dip galvanized or electro-galvanized steel sheet. In the case of the technology, the cohesive failure of an adhesive where the sealer is peeled off in an impact peel test and a shear strength test does not occur after the adhesion of the steel sheet by the sealer, but the cohesive failure of a coated steel sheet in which a Zn—Fe coating layer is peeled off may still occur due to the alloy phase formed between Fe and Zn when zinc and a thin film coating material are subject to the alloying process.
The present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide a high-corrosion resistance zinc alloy coated steel sheet having good sealer adhesion and corrosion resistance.
Also, it is an object of the present invention to provide a process of manufacturing a zinc alloy coated steel sheet having good sealer adhesion and corrosion resistance.